Summary: Macrophages play a critical role in the pathogenesis of HIV infection, both as targets for virus replication and as sources of multifunctional cytokines. Our early studies showed that normal human monocytes stimulated with the HIV-1 envelope protein, gp120, produce the HIV-modulatory cytokines, TNF-a, IL-1b, IL-6 and GM-CSF (JI 1991) and the potent vasoactive peptide, ET-1 (JI 1993). Our studies with monocyte derived macrophages (MDM) revealed that HIV-1 infection fails to induce these latter cytokines in vitro, but consistently induces M-CSF production with a kinetics paralleling virus replication (JI 1995). M-CSF facilitates differentiation and increases the susceptibility of MDM to HIV infection by enhancing expression of the HIV receptors, CD4 and CCR5. We determined the viral specificity of M-CSF production in MDM using a panel of single-stranded RNA viruses, including HIV-1, HIV-2, measles and respiratory syncytial viruses (MV & RSV) and found that only monocytropic strains of HIV-1 (JI 2000) and HIV-2 caused enhanced production of M-CSF. Chemokines, such as MIP-1a/b and MCP-1, were also produced by HIV-1 infected MDM, but not HIV-2, suggesting that both chemokines and M-CSF are needed to establish a viral reservoir or that chemokine production may correlate with pathogenicity (in prep, 2001). Our other studies using anti-retroviral agents (AZT and Ritonavir) suggest that virus replication and M-CSF production are inextricably linked in HIV-infected MDM, such that inhibition of one leads to comcommitant inhibition of the other (AIDS, submitted). Since HIV arose from cross-species transmission of SIV and progressed from a benign to highly pathogenic disease with initial infections targeting MDM, we are investigating the ability of SIV to infect primary human macrophages and induce M-CSF and other cytokines or chemokines. Using primary isolates, we are finding that SIV from multiple lineages are able to replicate in human MDM. Ongoing studies will determine receptor usage and cytokine production to obtain insight into pathogenic determinants of these lentiviruses for the human population with the hope of identifying novel targets for therapeutic intervention. Since astrocytes surround HIV-producing cells in the brain, which might influence virus replication, we studied their effect on HIV expression in human MDM. Co-culture of HIV-infected MDM with primary human astrocytes resulted in reduced HIV replication mediated in part by an unidentified astrocyte secreted factor (AIDS 1999) in addition to expression of inducible nitric oxide synthase (iNOS) and production of NO by astrocytes (Blood 1999). Our data suggest that astrocytes play a pivotal role in determining the course of neurologic HIV disease via production of HIV modulating cytokines and expression of iNOS/NO. It also leads us to speculate that neurologic damage observed in HIV disease may ensue from prolonged, high level production of NO by astrocytes, which may reflect a host attempt to inhibit virus replication.